Melatonin (N-acetyl-5-methoxy tryptamine), is a substance found in all animals as well as in plants, fungi, and bacteria. Melatonin is a powerful antioxidant and maintains the body's circadian rhythm. At least in vertebrates, melatonin is biosynthesized from the native metabolite L-tryptophan via the intermediates 5-hydroxy-L-tryptophan (5HTP), serotonin (a.k.a. 5-hydroxytryptamine) and N-acetylserotonin, with the conversion of serotonin to N-acetylserotonin being catalyzed by AANAT (EC 2.1.3.87).
Klein et al. (2007) provides an overview of the biological chemistry of AANAT, describing, e.g., that AANAT family members have been found in Gram-positive bacteria, fungi, algae, cephalochordates, and vertebrates. Further, Liu et al. (2014) identified a biosynthetic gene cluster in Streptomyces griseofuscus encoding eight proteins catalyzing the biosynthesis of physostigmine from 5-hydroxytryptophan. Based on in vitro assays, one of the proteins, PsmF (SEQ ID NO:1), was reported to function as a (serotonin) N-acetyltransferase.
Over-the-counter dietary supplements based on chemically synthesized melatonin have been available for many years in the U.S. Melatonin can also be produced in microbial cells engineered to express an appropriate biosynthetic pathway (see, e.g., WO 2013/127915 A1, WO 2015/032911 A1 and US 2014/134689 AA). For cost-effective production of bulk chemicals in microbial hosts, however, high product titers are essential. Accordingly, there is a need for optimizing AANAT activity in microbial host cells, for production of N-acetylserotonin, melatonin and related chemicals.
The present invention addresses these and other needs in the art.